It would be a real help, as a freelancer, if 'mouse over' a file title the client included with an offer would yield a thumbnail (say, 200x200) - or a direct click on the title would give that thumbnail.
Then it would be possible to immediately see what is involved, and let the freelancer know whether it would be worthwhile to take the time to download the file itself.
Just a suggestion.
If you are receiving a file along with an offer, chances are the file is important to your project and the deliverable.
If you don't trust the client, then you would reject the offer. Let's have the Upwork team focus on products that benefit the client and freelancer community.